Solid electrolytic capacitors comprise an anode body of a valve metal such as Al (aluminum) or Ta (tantalum), a dielectric oxide film formed on the surface of the anode body by an electrolytic oxidation treatment, and a cathode layer formed by applying an electrically conductive substance, such as electrolyte, MnO2 (manganese dioxide) or conductive organic compound, to the oxide film in intimate contact therewith. The term the "valve metal" as used herein refers to a metal which forms a highly compacted durable dielectric oxide film when subjected to an electrolytic oxidation treatment. Such metals include Ti (titanium) and Nb (niobium) in addition to Al and Ta. Since the dielectric oxide film has a very small thickness, electrolytic capacitors have the advantage that they can be smaller in size and greater in capacity than other paper capacitors and film capacitors.
Electrolytic capacitors wherein a solid conductive substance, such as MnO2 or conductive organic compound, is used for the cathode layer are called solid electrolyte capacitors. Examples of such conductive organic compounds are polypyrrole, polyaniline and like conductive high polymers, and TCNQ (7,7,8,8,-tetracyanoquinodimethane) complex salts.
These conductive organic compounds are higher than electrolytes and MnO2 in electric conductivity. Accordingly, the solid electrolyte capacitors wherein the conductive organic compound is used for the cathode layer are lower in ESR (equivalent series resistance) and more excellent in high-frequency characteristics than when an electrolyte or MnO2 is used for the cathode layer. These capacitors are presently used in various electronic devices.
As a process for preparing the cathode layer from the conductive high polymer among the above-mentioned conductive organic compounds, it is known to utilize chemical oxidative polymerization or electrolytic oxidative polymerization. Chemical oxidative polymerization is a process wherein a monomer is oxidatively polymerized with use of an oxidizing agent to prepare a high polymer. Electrolytic oxidative polymerization is a process wherein an oxidation reaction occurring on the anode in electrolysis is utilized to subject a monomer to oxidative polymerization and form a high polymer on the anode.
The process resorting to chemical oxidative polymerization comprises applying an oxidizing agent to the dielectric oxide film, and bringing the oxidizing agent into contact with a solution or gas of the monomer to be made into a conductive high polymer to oxidatively polymerize the monomer, whereby a conductive high-polymer layer is formed on the dielectric oxide film. However, the conductive high-polymer layer formed by this process has the drawback of being low in strength, liable to develop irregularities and lower in electric conductivity than the conductive high-polymer layer formed by electrolytic oxidative polymerization. The process therefore fails to provide a cathode layer which is fully satisfactory for use in high-performance solid electrolyte capacitors.
On the other hand, electrolytic oxidative polymerization, when resorted to, generally affords a uniform conductive high-polymer layer having a high strength, high electric conductivity and satisfactory quality, whereas when the conductive high-polymer layer is to be formed directly on the dielectric oxide film by electrolytic oxidative polymerization, the oxide film, which is an insulator, fails to function as an anode, making it impossible or extremely difficult to form the high-polymer layer on the oxide film.
Accordingly, it has been proposed to form a first cathode layer on the dielectric oxide film by a process other than electrolytic oxidative polymerization and to subsequently effect electrolytic oxidative polymerization with the first cathode layer serving as an anode to thereby form a second cathode layer of conductive high polymer on the first cathode layer.
JP-B-74853/1992 filed for Japanese patent application by Japan Carlit Co., Ltd. (U.S. Pat. No. 4,780,796 with priority claim based on the patent application), and JP-B-65009/1991 and JP-B-23410/1992 of the same company, and JP-B-83167/1993 filed for Japanese patent application by Nippon Chemi-Con Corp. disclose solid electrolyte capacitors wherein a conductive high-polymer layer is formed as the first cathode layer by chemical oxidative polymerization. JP-B-67767/1992 filed for Japanese patent application by Matsushita Electric Industrial Co., Ltd. discloses a solid electrolyte capacitor having an MnO2 layer as the first cathode layer. Japanese Patent Application 164019/1997 not laid open and filed conjointly by Sanyo Electric Co., Ltd. and Sanyo Electronic Components Co., Ltd., the assignee of the present patent application, discloses a solid electrolyte capacitor wherein a layer of TCNQ complex salt is formed as the first cathode layer.
FIG. 6 shows the common step of forming a second cathode layer from a conductive high polymer on the first cathode layer by electrolytic oxidative polymerization. An electrolyte 51 is placed in an electrolytic bath 50. The electrolyte 51 contains a monomer capable of forming a conductive high polymer, and a supporting electrolyte for giving a desired electric conductivity to the electrolyte 51. An anode body 1 formed with a dielectric oxide film and a first cathode layer is immersed in the electrolyte 51. Next, an external electrode 9 is held in contact with the first cathode layer 3 of the anode body 1, and a positive voltage is fed to the external electrode 9. The positive voltage is fed to the first cathode layer 3 in contact with the external electrode 9, causing an oxidation reaction, whereby the monomer is oxidatively polymerized into a conductive high polymer. Thus, the second cathode layer of conductive high polymer is formed on the first cathode layer 3.
In producing solid electrolyte capacitors actually, a multiplicity of anode bodies 1 are immersed in the electrolyte 51 as one lot and subjected to electrolytic oxidative polymerization at the same time to form the second cathode layer on each anode body. When anode bodies 1 are subsequently subjected to electrolytic oxidative polymerization as another lot, it has been found that the electrolytic capacitors of the subsequent lot are higher in ESR and lower in high-frequency characteristics than those of the previous lot. For this reason, it is conventional practice to replace the electrolyte by a fresh one every time the polymerization operation is conducted for one lot of anode bodies 1. This entails an impaired operation efficiency and an increased cost.
Further when a current is fed through the external electrode 9 in contact with the first cathode layer 3 for electrolytic oxidative polymerization, the current density fails to remain constant depending on the degree of contact between the electrode 9 and the first cathode layer 3, presenting difficulty in forming a uniform second cathode layer. Moreover, when the external electrode 9 is removed after the second cathode layer has been formed, the second cathode layer becomes dislodged locally, causing damage to the dielectric oxide film at the same time and creating problems such as increased leakage current from the capacitor.
To avoid such problems, JP-A-283289/1993 filed for Japanese patent application conjointly by Elna Co., Ltd. and Asahi Glass Co., Ltd. discloses an external electrode 9 having a curved end 91 for use in feeding by contact with the first cathode layer 3. The electrode thus designed precludes a great mechanical stress from acting locally on the anode body 1, obviating the problem that the second cathode layer is partly separated off by the removal of the external electrode 9 on completion of polymerization.
The electrode thus designed nevertheless has a problem: since the current is fed to the first cathode layer 3 concentrically at a point P, there occurs a potential difference between the region in the vicinity of the feeding point P and a region away from this point, resulting in uneven growth of the second cathode layer in accordance with the potential distribution.
An object of the present invention is to provide a process for producing electrolytic capacitors wherein the electrolyte is repeatedly usable for electrolytic oxidative polymerization to form second cathode layers having a constant electric conductivity free of impairment.
We have found that when electrolytes are checked for pH before and after electrolytic oxidative polymerization, the polymerization greatly varies the pH. This variation is thought attributable to the phenomenon that the conductive high polymer produced is doped with a portion or the whole of the supporting electrolyte. Since the electric conductivity of conductive high polymers are dependent generally on the pH of the polymerization system, we have contrived the means to be described below to accomplish the object.
A second object of the invention useful for producing solid electrolyte capacitors is to improve the step of forming the second cathode layer by electrolytic oxidative polymerization with current fed to the first cathode layer and to provide a process and an apparatus for forming the second cathode layer with a thickness made uniform to the greatest possible extent.